Numerous techniques have been proposed in the past for the formation of composite articles wherein fiber reinforcement is provided within a matrix of a polymeric material. Heretofore such composite articles commonly have been prepared wherein the matrix is a highly cross-linked thermosetting resin. During the formation of such composite articles the fibrous material commonly is impregnated with a liquid comprising the neat or dissolved uncured or partially-cured thermosetting resin to form a pliable, tacky prepreg which is subsequently placed in the desired configuration and is cured to a predetermined solid form over an extended period of time. The drapable and tacky nature of such prepregs has greatly aided in their use when forming composite articles having a complex configuration since adjoining layers tend to adhere well to each other and stay in place during the composite formation procedure. However, the resulting composite articles which employ a thermosetting resin in the matrix phase commonly are somewhat brittle and are found to provide deficient mechanical properties for some end use applications where toughness is of importance.
Accordingly, discussions can be found in the technical literature concerning the desirability of forming fiber-reinforced composite articles wherein the matrix phase is a thermoplastic polymer which can be relied upon to impart different mechanical properties to the composite article. When attempts have been made to impregnate reinforcing fibers with a molten thermoplastic polymer, it commonly is found that the melt viscosity of the polymer is too great to enable uniform impregnation between adjoining fibers. Also, when the melt is allowed to cool the resulting article is stiff and boardy and lacks the desirable pliable nature of a prepreg formed while using a thermosetting resin. When attempts have been made to dissolve the thermoplastic polymer in a solvent and to impregnate the fibrous material followed by evaporation of the solvent, difficulties also commonly have been encountered. For instance, some thermoplastic polymers are not readily soluble, uniform impregnation has been difficult to achieve, and complete solvent removal commonly has been a problem.
Blends of reinforcing fibers and thermoplastic fibers have been proposed for use in the formation of composite articles wherein the thermoplastic fibers are melted to form the matrix phase. Such blends inherently lack tack which is useful during layup to form a fiber-reinforced composite article.
Reference also can be found in the literature to providing the matrix-forming thermoplastic polymer as small particles which are mixed with the fibrous material prior to composite formation. Heretofore, such proposals have not become a commercial reality primarily because of the inability to achieve uniform impregnation of the fibrous reinforcement, the tendency of the polymer particles to further segregate within the fibrous material and to separate from the fibrous material, and the necessity to melt the particles following placement among the fibers so as to immobilize the same. Such fusion has resulted in the formation of a stiff boardy product which lacks tack and is largely unsuitable for use in the formation of a composite article having a complex configuration. Also, when the impregnation is not uniform the polymer matrix will not be uniformly dispersed among fibers in the resulting composite article. This will result in a voidy product having resin-rich and resin-lean areas and unpredictable non-uniform mechanical properties.
British Patent No. 1,264,432 concerns the application of a dispersion of particles of a thermoplastic polymer to newly spun glass fibers.
British Patent No. 1,424,168 concerns the formation of a stiff sheet or tape prepreg wherein fibers are contacted with a bath containing water, thickening agent, and resin particles which immediately thereafter are melted at a temperature between 60.degree. to 100.degree. C. to cause the particles to adhere to the fibers. Uniform fiber impregnation would not be achieved and the stable tacky and pliable product of the present invention would not result.
U.S Pat. No. 4,292,105 concerns the impregnation of a fibrous material from a bath of the specified composition containing water, thickener, and polymer particles. The conditions described would not achieve uniform impregnation to produce a quality prepreg. Also, in the working examples the product would have no drapability since it was heated to fuse the polymer.
U.S. Pat. No. 4,626,306 concerns the impregnation of a fibrous lap with resin particles by dipping in a bath containing the particles in the absence of a binding agent. Uniform fiber impregnation would not be achieved and the stable tacky and pliable product of the present invention would not result.
U.S. Pat. No. 4,680,224 makes reference to the impregnation of fiber strands with a polymer powder or a slurry of powder. The patent is devoid of teachings concerning how such impregnation can be accomplished in the improved manner contemplated herein or how one could provide the improved product of the present invention.
It is an object of the present invention to provide a method for the formation of an improved fibrous material suitable for the formation of a substantially void-free composite article comprising a plurality of adjoining substantially parallel reinforcing fibers.
It is an object of the present invention to provide an improved method for impregnating a fibrous material with solid particles of a matrix-forming thermoplastic polymer in a uniform and consistently reliable manner.
It is an object of the present invention to provide an improved method for producing a fibrous product suitable for use in the production of composite articles having solid particles of matrix-forming thermoplastic polymer substantially uniformly dispersed between adjoining filaments in a uniform and stable manner in the absence of fusion bonding.
It is another object of the present invention to provide a method for producing an improved product containing solid particles of thermoplastic polymer substantially uniformly dispersed among adjoining filaments which is drapable and tacky at ambient conditions, is handleable without segregation of the particles within the fibrous material, and which upon the application of heat and pressure can be transformed into a substantially void-free composite article of a predetermined configuration.
It is a another object of the present invention to provide an improved fibrous material suitable for use in the formation of a fiber-reinforced substantially void-free composite article and which exhibits a combination of highly desirable characteristics as discussed herein including drapability, handleability without adverse consequences, and tack.
It is a further object of the present invention to provide an improved fibrous material suitable for use in the formation of high performance fiber-reinforced parts for use in aircraft, spacecraft, industrial machinery and automotive applications.
These and other objects, as well as the scope, nature and utilization of the present invention will be apparent to those skilled in the art from the following detailed description and appended claims.